Teach this class about "highest-level syntactic breaks" but only for

headers with no charset or 'us-ascii' charsets.  Actually this is only
partially true: we know about semicolons (but not true parameters) and
we know about whitespace (but not technically folding whitespace).
Still it should be good enough for all practical purposes.

Other changes include:

__init__(): Add a continuation_ws argument, which defaults to a single
space.  Set this to change the whitespace used for continuation lines
when a header must be split.  Also, changed the way header line
lengths are calculated, so that they take into account continuation_ws
(when tabs-expanded) and any provided header_name parameter.  This
should do much better on returning split headers for which the first
and subsequent lines must fit into a specified width.

guess_maxlinelen(): Removed.  I don't think we need this method as
part of the public API.

encode_chunks() -> _encode_chunks(): I don't think we need this one as
part of the public API either.